This invention relates to improvements in battery charging circuits. More particularly, this invention relates to a battery charging circuit in which the flow of charging current into the battery is controlled by a battery condition, such as, for example, battery temperature. The control which is effected is the automatic selection of either a high or low rate of charge as determined by the aforesaid battery condition.
It is a common practice in charging discharged batteries to provide a "fast" charge and then to reduce the charging current to a minimum steady flow, commonly known as a "trickle" current, when the battery approaches full strength. The reduction of the high charging current becomes imperative with regard to certain batteries because not only is permanent damage to the battery likely but in some cases, most notably that of nickel-cadmium (Ni-Cd) batteries, venting can occur, resulting in rapid deterioration of the battery.
With particular reference to a Ni-Cd battery for ease of description, the attainment of a full charge is accompanied by a sudden increase in battery temperature. This abrupt temperature change is a condition which has been used previously in battery charging circuits to terminate charging. As an example as to how this is accomplished, a thermostat can be thermally coupled to the battery to sense this temperature increase, and the thermostat contacts are connected electrically in series with a relay. Once a preset temperature level is attained, the thermostat contacts open and cause the relay to de-energize which action opens the charge path through which the fast charge of the battery was being effected. In this way the high charging current is eliminated and a trickle charge is then applied to the battery. The battery can now be removed from the charger and inserted into the desired battery-driven equipment, or if the charging circuit is built into the battery-driven equipment, an appropriate switch can be operated to transfer the battery from a charging state to an operational state.
While an arrangement as discussed above is useful under normal circumstances, there are disadvantages with this arrangement which have become apparent in practice. After the battery becomes charged, it often occurs that the battery is put into immediate operation and is discharged at a sufficient rate to keep the battery temperature above the level at which the thermostat contacts close. If this battery, having again become discharged, is immediately placed in the charging circuit, and a switch actuated to initiate what should be a fast charge, high current flow does not occur because the thermostat contacts are open. This then requires the user of the equipment to wait until the battery cools sufficiently for the thermostat contacts to close, and then to actuate the charging cycle to initiate the desired fast charge. If prior to the cooling of the battery, any attempt is made to override or bypass the thermostat in order to achieve a fast charging mode, there is as pointed out above, a strong likelihood of battery destruction or venting.